1. Field of the Invention
The present invention relates to a hydrometallurgical process for nickel oxide ore to leach out and recover nickel and cobalt from a nickel oxide ore by using acid under high temperature and pressure. The present application claims priority based on Japanese Patent Application No. 2013-056012 filed in Japan on Mar. 19, 2013, and Japanese Patent Application No. 2013-195610 filed in Japan on Sep. 20, 2013. The total contents of the Patent Application are to be incorporated by reference into the present application.
2. Description of Related Art
Conventionally, as a smelting process for recovering nickel and cobalt from a nickel oxide ore containing iron as a main component and having a nickel content of 1% to 2% by weight by separating nickel and cobalt from iron, for example, there has been performed a pyrometallurgical process by which a nickel oxide ore is roasted to reduce and sulfurize nickel components, and then, smelting is performed to produce a matte containing a nickel sulfide; or a reducing-roasting and leaching process by which a nickel oxide ore is reduction-roasted, and then, while ammonia complex ions are formed, nickel and cobalt are selectively leached out.
However, these smelting processes includes pyrometallurgical processing steps of drying and roasting a raw ore containing a large amount of free water, and furthermore, in the smelting processes, it is impossible to selectively reduce only nickel and cobalt, and therefore, there is a problem that the smelting processes lead to much waste in terms of energy and cost. Therefore, in opposition to these smelting processes, the development of a process capable of being performed simply and at low cost has been demanded.
In recent years, high pressure acid leach using sulfuric acid has been attracting attention as a hydrometallurgical process for nickel oxide ore. Unlike pyrometallurgical process, which is a conventional common smelting process for nickel oxide ore, this high pressure acid leach does not include pyrometallurgical steps, such as reduction and drying step, but comprises consistent hydrometallurgical steps, and thus, is advantageous in terms of energy and cost. Furthermore, the high pressure acid leach has an advantage that a nickel-cobalt containing sulfide whose nickel grade is increased to be up to approximately 50% by weight (hereinafter, also referred to as a “nickel-cobalt mixed sulfide”) can be attained.
This high pressure acid leach comprises, for example: an ore slurry preparation step (which corresponds to a “first step” mentioned below) of grinding a nickel oxide ore as a raw material to a predetermined size to form slurry; a leaching step (which corresponds to a “second step” mentioned below) of adding sulfuric acid to the ore slurry and performing a leaching treatment under high temperature of not less than 200 degrees C. and high pressure by an autoclave or the like to obtain leach slurry; a solid-liquid separation step of separating a leach residue and a nickel-cobalt-containing leachate in the leach slurry; a neutralization step of adjusting the pH of the leachate containing impurity elements together with nickel and cobalt to form neutralization precipitate slurry containing impurity elements such as iron and a purified mother liquor for nickel recovery; and a sulfurization step of supplying a sulfurizing agent such as hydrogen sulfide gas to the mother liquor for nickel recovery to form a nickel-cobalt mixed sulfide and a barren liquor (for example, refer to Patent document 1).
The high pressure acid leach has a great advantage because, in the leaching step, the oxidation-reduction potential and the temperature of a leachate in a pressure leaching reaction vessel are controlled, whereby a major impurity, iron, is fixed as a leach residue in the form of hematite (Fe2O3), and nickel and cobalt can be selectively leached out in contrast to iron. On the other hand, the high pressure acid leach has a problem that, without a roasting step, a leaching treatment is directly applied to nickel oxide ores which exhibit wide variations in ore composition, organic component content, and the like, and therefore, depending on particularly the amount of organic components contained in nickel oxide ores, oxidation-reduction potential (ORP) at the time of leaching widely varies.
For example, in the case where oxidation-reduction potential at the time of leaching is too high, chromium contained in a nickel oxide ore is leached out in a state where the chromium is oxidized to be hexavalent. In order to remove this hexavalent chromium in downstream steps such as a neutralization treatment step and a wastewater treatment step, the chromium needs to be reduced to be trivalent using a reducing agent, and accordingly, an increase in smelting cost is inevitable. Unless a reduction treatment is given, a problem arises that chromium is contained as an impurity in a nickel or cobalt product, or chromium remains in a post-waste-water-treatment solution. On the other hand, in the case where oxidation-reduction potential at the time of leaching is too low, a problem arises that titanium which is used for an autoclave as a corrosion-resisting material is deteriorated, and in addition, a high-temperature hydrolysis reaction of iron is inhibited, whereby a large amount of iron remains in a leachate, an increase in the amount of a chemical agent used in a neutralization treatment step as a downstream step and an increase in the coprecipitation amount of nickel and cobalt, that is an increase in loss is caused.
To solve these problems, for example, there is disclosed a process by which at least one of sulfur and a carbon compound is added to ore slurry and the oxidation-reduction potential (Ag/AgCl basis) of a leachate is controlled to 400 to 600 mV to perform leaching (for example, refer to Patent document 2). This process is such that added sulfur and an added carbon compound act as reducing agents, whereby the oxidation-reduction potential is decreased and controlled to not more than 600 mV, at which the elution of hexavalent chromium is not caused. It should be noted that, when the oxidation-reduction potential is less than 400 mV, not only a poor oxidation hydrolysis reaction of iron is caused, but also the corrosion resistance of facility materials is impaired, and therefore, the amount of sulfur or a carbon compound added is adjusted to control the oxidation-reduction potential to 400 to 600 mV.
Furthermore, there is also disclosed a process by which, without adding a reducing agent, the mixing ratio of ores having different amounts of sulfur and a carbon compound contained is changed, whereby a target oxidation-reduction potential (Ag/AgCl basis) is controlled to 400 to 600 mV (for example, refer to Patent document 3).
In a leaching treatment in a leaching step using the process disclosed in the foregoing Patent document 2, leached-out iron is oxidized to hematite and hydrolyzed, and therefore, it is indispensable that, normally, high pressure air is used as an oxidizer and a reaction vessel is kept pressurized. However, conditions for using high pressure air in this process have not been disclosed. From a viewpoint that variation in oxidation-reduction potential due to variations in ore composition and organic component content is dealt with to prevent particularly a decrease in “iron oxidation ratio” to trivalent iron due to a decrease in oxidation-reduction potential and the elution of chromium, this process aims to lower oxidation-reduction potential by making added sulfur and an added carbon compound act as reducing agents, and therefore, an excessive amount of high pressure air is blown in to control oxidation-reduction potential. As a result, the amount of exhaust gas from a pressurized reaction vessel increases, thereby causing an increase in heat loss, and, to compensate the heat loss and maintain the temperature of the pressurized reaction vessel, the amount of high pressure vapor used for temperature control increases, thereby causing an increase in energy cost.
Furthermore, in a leaching treatment in a leaching step using the process disclosed in Patent document 3, in the first step of preparing ore slurry, the mixing ratio of ores having different amounts of sulfur and a carbon compound contained is changed, whereby, mainly, a target carbon grade of solids in the ore slurry is adjusted in advance, and thus, an excessive amount of high pressure air is prevented from being blown in at the time of a leaching treatment in the second step. However, ores have different ore compositions and different organic component contents, and therefore, not only the mixing ratio is limited, but also, depending on the mixing ratio, an increase in magnesium grade and aluminum grades of solids in ore slurry is caused. These impurities such as magnesium and aluminum react with sulfuric acid which is added in acid leaching in the second step, thereby consuming the sulfuric acid, and accordingly, the leaching rate of nickel and cobalt is decreased. To avoid such decrease, an excessive amount of sulfuric acid needs to be added, but, in this case, there arise problems of an increase in the amount of a chemical agent used in the neutralization treatment step as a downstream step and an increase in the coprecipitation amount of nickel and cobalt.